白山黑水

白山 (Báishān, literally “white mountains”) refers to 長白山／长白山 (Chángbáishān, “Changbai Mountain”), while 黑水 (Hēishuǐ, literally “black waters”) refers to the Songhua (Sungari) River, historically referred to as 混同江. First attested in the History of Jin:

生女直地有混同江、長白山，混同江亦號黑龍江，所謂「白山、黑水」是也。 [Literary Chinese, trad.]

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生女直地有混同江、长白山，混同江亦号黑龙江，所谓「白山、黑水」是也。 [Literary Chinese, simp.]

From: 1344 CE, Toqto'a (lead editor), History of Jin

Shēngnǚzhí dì yǒu Hùntóngjiāng, Chángbáishān, Hùntóngjiāng yì hào Hēilóngjiāng, suǒwèi “báishān, hēishuǐ” shìyě. [Pinyin]

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白山黑水

1. (figurative) Northeast China